[Feature] Displaying next tasks suggested.

Fixes #5.

- Implemented detection of suggested tasks.
- Added models and test for Node.
- Testing for Task and node addition reciprocity.
- Replacing usage of custom nodes and tasks for these models in the app
- Replaced usage of generateNewTask with Task constructor.
- d3Service tests
- Refactored connectNodes, disconnectNodes outside of sugiyamaService.
- Refactored usage of nodes and tasks outside of services.
- Refactored taskDependencyService.removeFromDependencies into taskListService.removeTask
- Removed taskDependencyService.
- Added arrows to graph.
- Displaying next to complete tasks in graph.